Police say they were informed of the plans by the student's grandmother. According to KING-TV, his grandmother found a journal detailing his plans and called 911. She told police he kept a rifle hidden in a guitar case.

Authorities said that excerpts from the journal detailed his plan to shoot individuals and use home-made explosives to attack ACES High School.

Police arrested the student at school and found a knife and marijuana on him when they took him into custody.

Police executed a search warrant at his grandmother's home. Along with his journal and rifle, they also found inert grenades and other "evidentiary items."

The 18-year-old was booked on probable cause for attempted murder due to planning and taking substantial steps toward executing a school shooting.